I see a few things that may be causing problems.
To answer your question though, this line:
$loop = new WP_Query( array( 'post_type' => 'subjects', 'posts_per_page' => '3' ) );
is performing a query on a post type called “subjects”.
I think you want to be pulling your quizzes. So the line should be:
$loop = new WP_Query( array( 'post_type' => 'quiz', 'posts_per_page' => '3' ) );
I’d go through and check capitalization as well. When you registered your post type you called it “Quiz”, but later referred to it as “quiz”. It should be lowercase. You did this as well when you changed the CPT slug.